The present invention relates to an improvement in a colour sorting apparatus for sorting objects such as grains according to their colours, and more particularly, to a control device for automatically controlling a background brightness thereof.
A colour sorting apparatus is so arranged that the amount of reflected and/or transmitted light from grains of a usual colour passing through a sorting area and the amount of reflected and/or transmitted light from the background thereof are made to be identical to each other and are detected by means of a sensor consisting of such means as a photo-sensitive element, and for sorting grains, the sensor detects the variation in the amount of light which occurs at the time when a grain with a colour different from the usual colour passes the sorting area and an ejector blows away the grain with a colour different from the usual colour on the basis of the signal detected by the sensor. When this colour sorting apparatus is continuously operated for a long period of time, the background becomes dusty so that a disagreement develops between the amount of reflected and/or transmitted light from the grains and the amount of reflected and/or transmitted light from the background, thereby making it impossible to carry out the desired sorting operation. Thus, in a conventional apparatus, the signal from the sensor is displayed on an oscilloscope and the background brightness is manually controlled in order to achieve the desired sorting performance. However, such manual control of background brightness required for the conventional apparatus involves troublesome and time-consuming work, in particular, for the conventional apparatus which is provided with a large number of sorting channels.